This invention relates to methods for passivating the surfaces of semiconductors. More particularly, this invention relates to methods for stabilizing such passivated surfaces from the deleterious effect of oxidation for compound semiconductors.
It has been demonstrated for most III-V compound semiconductors that sulfidation of the surface results in improved electronic properties through the reduction of mid-gap surface states that promote rapid surface carrier recombination. Treatment with selenium has also been investigated and has been shown to sometimes work somewhat better than sulfur. Calculations for tellurium on GaAs have suggested that tellurium, in the chalcogen family with sulfur and selenium, would not have the same effect on mid-gap state as S and Se and has not been used for this purpose. High surface recombination velocities and/or Fermi-level pinning due to a high density of mid-gap surface states (&gt;10.sup.12 /cm.sup.2) have diminished the performance of heterojunction bipolar transistors (HBTs) and delayed the realization of metal-insulator-semiconductor (MIS) devices in III-V compound semiconductors. Reaction of the GaAs (or other III-V compound semiconductor) surface with sulfur or its compounds produces a dramatic decrease in the interface states responsible for surface recombination and Fermi-level pinning. This sulfidation has been accomplished in a number of ways including immersion in aqueous Na.sub.2 S and/or NH4Sx solutions, anodic sulfidation, treatments with polythiols, exposure to H.sub.2 S that has been thermally or plasma activated, and photosulfidation. The following references are of interest in these areas and are incorporated by reference herein in their entirety: C. J. Sandroff, R. N. Nottenburg, J. C. Bischoff, and R. Bhat, Appl. PHys. Lett. 51, 33 (1987) (treatment of HBTs with Na.sub.2 S, instability of the S-treated surface); E. Yablonovich, C. J. Sandroff, R. Bhat, and T. Gmitter Appl. Phys. Lett. 52, 439 (1988)(treatment with NH.sub.4 S, Li.sub.2 S, Na.sub.2 S): J. F. Fan, H. Oigawa, and Y. Nannichi, Jpn.J. Appl. Phys. 27, L1331 (1988) (treatment with (NH.sub.4).sub.2 S.sub.x); S. Shikata, H. Okada, and H. Hayashi, J. Appl. Phys. 69, 2717 (1991)((NH.sub.4).sub.2 S.sub.x suppression of emitter size effect on beta of HBTs; and J. S. Herman and F. L. Terry, J. Vac. Sci. Technol. A11, 1094 (1993) (treatment with H.sub.2 S plasma). A preferred process is the photosulfidation process taught in U.S. Pat. No. 5,451,542 that utilizes UV photodissociation of sulfur vapor to form reactive S species that react with a semiconductor surface that has had the native oxide removed.
Unfortunately, the major problem with all these approaches has been the instability of the sulfided surface with respect to oxidation when exposed to air. Selenided surfaces are somewhat better but suffer from oxidation effects as well. As these surfaces re-oxidize, the density of mid-gap states rapidly returns to its original level. There have been at least two attempts to passivate these sulfidated surfaces against re-oxidation: a glow discharge in sulfur vapor with the GaAs heated to 400.degree. C., X.Hou, X. Chen, Z. Li, X. Ding, and X. Wang. Appl. Phys. Lett. 69, 1429 (1996), and immersion in S.sub.2 Cl.sub.2 /CCl.sub.4, X. Cao, X. Hou, X. Chen, Z. Li,R. Su, X. Ding, and X. Wang, Appl. Phys. Lett. 70, 747 (1997). Despite their air stability, these processes are limited for actual device applications by either the high temperatures employed or the over-etching of GaAs by S.sub.2 Cl.sub.2, which necessitates in-situ measurement of current gain to terminate the etch process at maximum device performance.
There remains an unmet need in the art for a process that easily provides for a stable passivation of a low-surface-state-density surface of a III-V semiconductor that is resistant to the deleterious effects of re-oxidation in air.